1. Field of the Invention
The present invention relates to a recording liquid for ink printers and a method for recording images, and in particular relates to a recording liquid for ink printers.
2. Discussion of the Related Art
As an output device of low running costs and of easy color printing for information equipment such as computers, ink printer systems are attracting public attention. Such systems include an ink-jet system, which is a method of image recording for recording images on recording media such as paper by discharging droplets of a recording liquid from a head, or an acoustic ink printing system as disclosed in Japanese Patent Publication No. 6-102378 (1994). Heretofore, as a recording liquid for ink-jet printers, an aqueous solution of a dye mainly containing water and a dye has been used. However, such a solution has problems in that when the recording liquid discharged from the nozzle arrives at a recording paper, the solution bleeds on the recording paper and produces dots significantly larger than the droplets of the recording liquid formed when discharged from the nozzle, resulting in the low density of recorded images, and the low image quality. Also, since the water resistance of recorded images is low, images are easily blurred or flowed by water; and since the light resistance of recorded images is low, images are easily discolored by the irradiation of sunlight or the like.
In order to solve these problems of dye aqueous solutions, recording liquids for ink-jet printers that contain film forming resin particles have been disclosed. For example, a recording liquid containing latex as resin particles is disclosed in Japanese Patent Publication No. 60-32663 (1985); a recording liquid that contains a resin having carboxyl groups and nonionic hydrophilic groups dispersed in water is disclosed in Japanese Patent Laid-Open No. 5-239392 (1993); a recording liquid that contains vinyl polymers with a non-cross-linking structure is disclosed in Japanese Patent Laid-Open No. 5-255628 (1993); and a recording liquid that contains polyester particles having ionic groups is disclosed in Japanese Patent Laid-Open No. 6-340835 (1994). Also, a process of compounding fine particles of a resin such as polyester, and a cross-linking agent, and cross-linking the resin on a recording medium is disclosed in Japanese Patent Publication No. 7-47355 (1995); and a process that uses water-soluble dyes and ultraviolet rays is disclosed in Japanese Patent Laid-Open No. 10-305570 (1998). Furthermore, the inventors of the present invention disclosed a recording liquid that contains pigments and fine particles of two or more resins in Japanese Patent Laid-Open Nos. 10-88045 (1998) and 10-88056 (1998) and in U.S. patent application Ser. No. 08/925,391 that will be issued soon. (U.S. patent application Ser. No. 08/925,391 that will be issued soon)
However, any of the recording liquids disclosed in above-mentioned Japanese Patent Publication No. 60-32663 (1985), Japanese Patent Laid-Open Nos. 5-239392 (1993), 5-255628 (1993), and 6-340835 (1994), and Japanese Patent Publication No. 7-47355 (1995) starts to form a film of the fine particles of resins following evaporation of moisture in the recording liquid due to contact with air at the discharging nozzle of the recording head and clogs at the discharging nozzles so that it cannot be discharged from the nozzles stably. Also, the bleeding of the recording liquid due to the capillary action into the fibers of recording paper cannot be completely prevented, and high-quality images cannot be obtained. Furthermore, the permeation of the recording liquid into recording paper cannot be completely prevented, and these recording liquids have limitation in the formation of high-density, high-quality images. Similarly, there is limitation in water resistance. When the above-disclosed recording liquid is diluted by water to prevent clogging at a discharging nozzle, the content of resin particles contained in the same volume of the droplets of the recording liquid, that is, the resin solid content contributing to image formation, decreases, resulting in significant lowering of image density, and high-quality images cannot be obtained. By the use of the above-disclosed conventional recording liquids, an increase in image density to achieve high-quality images by increasing the content of resin particles is incompatible with the prevention of discharging nozzle clogging. Although a process that uses water-soluble ultraviolet-sensitive dyes, and radiates light (ultraviolet rays) onto the dyes to make them insoluble and fix them is disclosed in Japanese Patent Laid-Open No. 10-305570 (1998), the water resistance and light resistance of the images are not satisfactory because of the use of dyes. Furthermore, since light is irradiated onto colorants, i.e. dyes themselves, the color images are easily changed by aging due to natural exposure to sunlight or the like. Although an increase in image density to achieve high-quality images by increasing the content of resin particles (increase in resin solid content) is compatible with the prevention of discharging nozzle clogging by the use of recording liquids disclosed in Japanese Patent Laid-Open Nos. 10-88045 (1998) and 10-88056 (1998), the stability of discharging must be improved when a large number of sheets are printed using a practically used ink printer.
The present invention has been made in view of the above circumstances and provides a recording liquid for ink printers that can obtain water-resistant high-quality images with high image density, and may not bleed on nor permeate into recording paper, and can be discharged continuously and stably almost without nozzle clogging. The present invention also provides a method for recording images by the use of such a recording liquid.